Driving from Chicago to Denver on I-90, what are sites to see on the way?

Millenium Park: you have to see it.

Theaters large and small: North Light Repertory, Steppenwolf, check the weekly listings. Some of the most creative and wonderful theater companies anywhere in the world thrive in Chicago.

Museums: you can never see enough of the major museums in Chicago: The Art Institute of Chicago, Field Museum of Natural History, Museum of Science and Industry. Really some of the best in the world.

There's entire books on the City. So much to say, so much to do.

I know this is an old question, but I saw it come up under South Dakota and I was just there, so I figured I'd write down a few places.

Of course, Mt Rushmore is one of the main attractions. Crazy Horse Memorial seems expensive because it's privately owned, but they do a good job explaining how admission is funding ongoing work. The eventual carving will be way bigger than Mount Rushmore.

If someone is reading this later and you go to Custer State Park, make sure you stop in the Visitor Center and check the location of the buffalo figures on the map. We didn't know this until afterwards, but that actually tells you where the herds are. Otherwise, it's nice scenery but you can drive the entire Wildlife Loop and not see hardly any buffalo.

We stayed in Rapid City which is the main town. Note that you may not get any internet signal on your phone as you approach Mount Rushmore. I had to reboot my phone to get it back once we returned to Rapid City. There are a bunch of bronze statues of U.S. Presidents in downtown Rapid City that are fun to see.

For kids, the WaTiki Indoor Waterpark Resort is good if it's hot outside. There are a few hotels that connect to it, so if you stay in one of those hotels you get discounted admission and it's easy to walk in and out of the waterpark to your room.

The other thing we thought was interesting was America's Founding Fathers Exhibit which has a 3-D recreation of the signing of the Declaration of Independence with wax figures.

Next to that is Reptile Gardens. If you go there, check out the crocodile show and the huge tortoises. You can walk up and touch them (the tortoises, not the crocodiles).

Many of these places offer the ability to visit again for free, so you can pop into Reptile Gardens, then come back later in the week, or your parking pass for Mount Rushmore is good for an entire year (but they log the license plate so it has to be the same vehicle).

You'll want to see the Corn Palace and Wall Drug Store plus the Badlands National Park.

While you're here to see Mt Rushmore there is much more to see. Crazy Horse Memorial in Custer, Sylvan Lake State Park and the Needles Hwy is a must. Also Custer State Park and of course Deadwood and Spearfish. Much more any rest stop with visitor info will have a lot of info. Oh also the Badlands National Park just off of I- 90 near Wall SD. Enjoy, plan a few days here! GODS COUNTRY😊

What you see depends on how long you take to get there. Madison (Wisconsin) is great—wander State St and the UW campus. If it’s Saturday morning, check out the farmer’s market around the Wisconsin State Capitol building. Loads of eateries around State Street.

As someone has already mentioned, Wisconsin Dells is touristy, but fun; decide in advance what you want to see/do there. It’s crowded in the summer.

Be sure to drive from Wisconsin Dells to the Mississippi River in the daylight. If you have time, gaze at the river from downtown La Crosse (Wisconsin) or scenic Apple Blossom Drive in La Crescent, MN. La Crosse also offers a couple of short Mississippi River cruises: http://www.lacrossequeen.com and http://www.mississippiexplorer.com (La Crosse isn’t listed, but call to see if that’s a mistake).There’s plenty of bluff-top hiking in the area and some nice places to eat in La Crosse/Onalaska.

The scenery from Wisconsin Dells to the Mississippi River is pretty, but after that the land starts flattening out. Lots of farmland.

Rochester (Minnesota) is famous for the Mayo Clinic, but there’s not much to see there. Stop at the Spam Museum in Austin MN (seriously, it’s fun).

As others said, visit the Mitchell Corn Palace and Wall Drug Store. Both stops are less than an hour.

I’d be happy to answer questions about stops in Wisconsin.

Your route, as you have it now, takes you right past the Wisconsin Dells.  Yes, it's a bit of a tourist trap but the Dells themselves are quite nice.  They have all kinds of boat rides that will take you past some of the scenery.  The House on the Rock and Taliesin are both in Spring Green WI, I have visited the House on the Rock, Taliesin always seems too expensive to me, House on the Rock, depending on your point of view, can be interesting or tacky.  "Loving Frank" by Nancy Horan gives some of the history of Taliesin, it's a novel but I believe a well researched one.  Madison (Wisconsin) is the capital of Wisconsin.  If you are into biking, there's an interesting old railroad trail from Sparta to Elroy, you can rent bikes at various points.